FAQ: Radiation Risk From Medical Imaging

March 31, 2010 -- In a sometimes contentious two-day meeting, the FDA has put forward its plan to reduce unnecessary radiation exposure from CT scans, nuclear medicine studies, and fluoroscopy.

These medical imaging techniques represent only about a fourth of imaging tests that expose U.S. patients to radiation, but they expose patients to nearly 90% of the radiation they get from medical imaging.

And more and more Americans get these tests every year. They're the main reason why the U.S. population's radiation exposure has almost doubled over the last 20 years.

An individual's chance of getting cancer from a single scan is small. But because the scans are so widely used, they cause a considerable amount of harm. One study estimated that the CT scans performed in 2007 are related to some 29,000 future cancers.

What are these tests? What are their risks? When do the tests' benefits outweigh their risks? Here are WebMD's answers to these and other questions.

What are CT scans, nuclear medicine studies, and fluoroscopy?

In a normal X-ray, a person gets a blast of radiation that creates an image on a two-dimensional square of film.

During a computed tomography or CT scan (sometimes called a CAT scan), a rotating device shoots X-rays through the body to produce several cross-sectional images. A computer assembles these images into a 3-D image of the inside of the body. Scans that take more images -- and expose the patient to more radiation -- yield sharper images.

During nuclear medicine studies, such as positron emission tomography or PET scans, the patient is given a small amount of a radioactive substance. A detector then views an image of this "radiotracer" as it moves through the body.

During fluoroscopy, a device passes continuous X-rays through the body to yield a real-time moving image.

How much radiation does a person get from medical imaging studies?

Getting a CT scan gives a patient as much radiation as 100 to 800 chest X-rays.

Getting a nuclear medicine study exposes a patient to as much radiation as 10 to 2,050 chest X-rays.

Getting a fluoroscopic procedure exposes a patient to as much radiation as 250 to 3,500 chest X-rays.

For perspective, a person gets the equivalent of one chest X-ray from normal background radiation in about two and a half days. In 2.7 years, people get as much radiation just from being on the planet as they do from an abdominal CT scan.

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What are the benefits of medical imaging procedures?

Medical imaging has vastly reduced the need for exploratory surgery and greatly improves the efficiency of many kinds of surgical procedures. These images have revolutionized the diagnosis of many diseases and conditions, have improved treatment planning, and save lives via image-guided therapies.

Even though the scans are expensive, they can reduce costs by making hospital stays shorter.

What are the risks of medical imaging procedures?

Even when done properly, medical imaging procedures can damage DNA and increase a person's lifetime risk of cancer.

In the U.S., a person has a one-in-five lifetime risk of cancer. Imaging can increase this risk.

For example, a 2009 study estimates that a CT coronary angiography given at age 40 will cause cancer in one in 270 women and in one in 600 men. The risk is about twice as high for a CT scan given at age 20 and about half as high for a CT scan given at age 60.

But scans aren't always given properly. Without taking the right precautions, a patient may be exposed to too much radiation, thereby increasing the risk without increasing the benefit.

Lack of standardization compounds the problem. For example, another 2009 study found that in the San Francisco Bay area alone, the dose of radiation given in the same kind of CT scan varied 13-fold between the highest and lowest dose given by different hospitals.

Moreover, doctors may prescribe scans that aren't medically justified. And since risk from radiation exposure accumulates over a lifetime, certain scans may not be appropriate for people who've already had a lot of scans.

Cancer isn't the only risk. Accidental exposures to high doses of radiation can cause burns and hair loss. Cataracts can develop in eyes directly exposed to radiation.

What is the FDA doing to reduce the risks of medical imaging?

The FDA's basic plan is twofold: make sure that every scan is medically justified and optimize the radiation dose a patient gets in each procedure.

To accomplish this, the FDA is launching three initiatives.

The first initiative will require the makers of CT and fluoroscope devices to incorporate safeguards into the design of their machines and to provide better training. Ideas include:

Requiring the devices to display, record, and report settings and radiation dose

Requiring the devices to alert users when the radiation dose exceeds the optimal dose for most patients

Enhanced training and certification of device users

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The second initiative will focus on giving doctors the tools they need to use medical imaging more wisely. Ideas include:

Requiring devices to transmit radiation dose information both to the patient's medical record and to a national dose registry.